Aurelien Tchouameni's long-range strike earned Real Madrid a 1-0 victory over Copa del Rey runners-up Mallorca on Saturday to send Carlo Ancelotti's side 11 points clear at the top of La Liga.
Players boarded a barge known as the 'Gabarra' and sailed 13 kilometres down the river from the affluent town of Getxo to the Bilbao city hall, where the party continued.
Athletic have been through years of heartache, having lost their previous six consecutive Spanish cup finals and a Europa League final to Atletico Madrid in 2012.
Bilbao are the second most successful team in the history of the Copa del Rey, their 23 titles behind only Barcelona's 31, their most recent coming in 2020-21.
Sergi Darder fired the decisive spot-kick as Real Mallorca beat Real Sociedad 5-4 on penalties to book their place in the Copa del Rey final on Tuesday.
Inaki Williams raced back from Africa to face Barcelona in the Copa del Rey and sent Athletic Bilbao into the semi-finals by scoring a crucial goal in their 4-2 extra-time triumph on Wednesday.
Defeat snapped Madrid's 21-match unbeaten run.
"It's a positive result, our play was positive in many moments, and we're in the last 16," Barca coach Xavi told Movistar.
Atletico Madrid progressed with a 3-1 win at third-tier Lugo after Memphis Depay struck twice, while Real Betis were knocked out in a 1-0 defeat by Alaves.
Barcelona forward Lionel Messi is out of hospital and recovering after a non-surgical procedure to cure his kidney problems.
Barcelona have confirmed that Lionel Messi is undergoing further tests on kidney problems which first emerged in December but expect the forward to return to training on Wednesday.
First-half goals from Munir El Haddadi and Neymar give Barcelona a slender lead in their Copa del Rey quarter-final tie against Athletic Bilbao with a 2-1 first-leg win.
Luis Suarez's hat-trick helps Barcelona extend their dominant La Liga record against Athletic Bilbao with a crushing 6-0 victory over the 10-man Basque side at Camp Nou.
Munir El Haddadi score both goals as Barcelona beat Espanyol 2-0 on Wednesday to ease into the Copa del Rey's quarterfinals with a 6-1 victory on aggregate.
Barcelona FC's appeal against their Uruguayan striker Luiz Suarez's two-game ban in the Copa del Rey fails as the decision is upheld by the Royal Spanish Football Federation (RFEF).
The Spanish Sports Press Association "demands a public retraction" of the statement made by Barcelona player Dani Alves where he branded the Spanish press as "f****** rubbish" on social media.
Espanyol goalkeeper Pau Lopez insists he did not deliberately attempt to injure Lionel Messi during his side's defeat to Barcelona.
Barcelona open up a comfortable lead in their Copa del Rey last-16 clash with Espanyol after winning the first leg 4-1 at Camp Nou.
Javier Mascherano has won 16 major trophies in his career, but he still couldn't avoid being nutmegged by Lionel Messi during training in an open session.
